Dolby 700 - High speed and reverse?

[David Ferguson]

(This may be a silly question, but I'm very new to all this, and just interested in film as a hobby, so I'm always learning - but also having fun!)

I recently got a lovely Kinoton FP38EC, that has the original Kinoton white light sound reader, and a Dolby Cat. 700 reader bolted on the side. The projector can go up to 300fps both forwards and backwards, however I'm wondering if the 700 reader can cope at this speed without any issues? And can it work in reverse? Or should I be unthreading the film from that before I either go in reverse, or high speed?

Obviously I'm not expecting it to be able to read the soundtrack when it's not going 24fps forwards, I'm just wondering it mechanically the reader will be okay, and if the film travelling through it will be okay too.

[Sam Chavez]

The Dolby digital readers and the electronics cannot read the soundtrack in reverse even at 24FPS. I would try to see how the 700 does at high speeds with some junk film but I expect it will not be able to cope with high speeds either forward or reverse. There is a fairly heavy flywheel in the reader and they are purely mechanical so doubt they would deal well with changes in direction or speed. I would bypass the reader except for normal forward operation. In any case, be gentle.

[Steve Guttag]

Going reverse isn't a concern. My concern with 300fps speed (either direction) is how fast you ramp it and if the drum has ANY slipping on the film. Since the drum is not recessed at all, any slipping will translate into scratches. You'll note that the "studio" RSSD soundhead has a magnetically coupled flywheel that decouples when the projector shuttles. This protects the film. The clutch only then recouples when the film speed matches the drum speed (forward projection at "sound speeds.)

I've done fwd/reverse at 100fps without issue on the likes of DTS readers and, if you are REAL careful on the SDDS reader (has to be a slow ramp up). Back with the AFI/Silver was running film, their reverse was set to 50fps since that would ensure that a reel was always rewound in time (more than 2X forward projection). They never had to unthread one of the auxiliary readers (DTS or SDDS...Dolby and Analog were via the RSSD "basement" readers).

[Leo Enticknap]

Dunno about high speed, but I do know that SR-D can be modified to work at low frame rates.

Many years ago (around 15-20, probably) I saw a demonstration at an archivists' conference of a modification to the system to enable SR-D to be used at frame rates as low as 16, on full-gate silent prints, for recorded orchestral soundtracks for silent movies. It involved a specially modified penthouse reader and custom firmware in a DA-20, and data blocks between the perforations on both sides of the film. For a demonstration, we were shown a reel of The Kid Brother at 20 FPS with the Carl Davis soundtrack, and it looked and sounded great. The obvious drawback, of course, is no fallback option if the digital decoding fails (though that didn't stop 70mm DTS from being used on a significant scale). A less obvious one is that for movies in which it is necessary to vary the frame rate during the show in order achieve a consistent, natural movement look throughout (e.g. Napoleon), this wouldn't work, because the SR-D track has to be mastered at a fixed frame rate.

But on a regular SR-D system on a projector with an adjustable transport speed, if you increase or decrease it by more than about a frame per second, the SR-D just drops out. I've worked at several arthouses that were so equipped. Once or twice, we had an SR-D show immediately following a silent one, and while running a rehearsal reel couldn't figure out why the SR-D wouldn't work. Then I saw that the motor was set to 22 or 18, reset it to 24, and the DA-20 or CP650 suddenly saw the track and started playing it. But even set to 23 or 25, nothing played.

[Sam Chavez]

Leo,

Were these houses that had the SR.D problem the same ones with modified DA20 software? I recall the software would play 11% fast and 5% (or the other way round) slow by design. This was in a large part done in answer to one of CDS' many fatal flaws in that It had to play at exactly 24 fps. Sync motors were not as widely used on 35mm projectors back then. The stunning incompetence not to know this was one factor in CDS failure. No analog backup was the core one, but a very shallow depth of field in the imaging lens meant even a chad of splicing tape would knock the film out of focus and unreadable. But I digress. Sorry.

[Victor Liorentas]

When does running at 300 fps become important? Why would it be needed?

[Dave Macaulay]

"When does running at 300 fps become important? Why would it be needed?"
The FP38EC is a disturbingly expensive studio machine, and the high speed is just for quick shuttling. Above some fps the intermittent sprocket stepper motor stops being intermittent and runs the film through without any stopping so you can't watch a 300fps image on screen, it's just a blur if you have the dowser open (and I think that automatically closes when the intermittent is just spinning). The ones I installed were for foley and dubbing studios.

[David Ferguson]

Exactly as Dave says, it's designed for studio use. I just use it for rewinding afterwards, which is why I was wondering whether I could do that with the film threaded through the Dolby Digital reader or not. It's obviously not a big deal to have to unthread the reader, but it sure would have been handy for automated purposes!

Steve - is the magnetically coupled flywheel part of the projector or the sound head? What I'm really asking is whether I can just get a Kinoton Reverse Scan Digital Reader and add that in, or does it need to be a studio one that does the magnetic coupling?

[Steve Guttag]

The studio RSSD was just that. A special version of the normal RSSD. It was only available for Studio versions of their normal soundhead.

Flywheel Accelerator.JPG

In addition to the magnetic clutch, there is that flywheel that has two rods for the hall effect sensor (to ensure the flywheel and film speed match) the flywheel accelerator to drive the flywheel up to speed. The clutch is pictured below:


image_202.jpg

This was all drive by a custom PCB, oddly located nowhere near the soundhead! Typically on the upper crossbeam towards the rear of the projector.

[Sean McKinnon]

As others have said the quick rewind speed is a life saver on dubbing stages where they may be running a short section of film over and over 10's or 100's of times. It has also come in handy on specialty installations where the projector may be synced to a light show and the film needs to rewind and advance to the starting position in a short amount of time.

[Mark Gulbrandsen]

Don't do it! I used to service a rock and roll system in Idaho that had a Dolby Digital reader on it. It has a good sized flywheel in it that the weight and or inertia of would cause damage to that sound head fairly rapidly and most certainly the film, especially in reverse. Being a tight loop sound head design, you would have to add solenoids to release any and all tension that's on the film when it's threaded normally. Only then it might work. But you would still see really fast wear on the digital readers aluminum lateral guide roller.

[Steve Guttag]

It should be noted, in agreeance with Mark, that Kinoton, for their 16mm soundhead on the FP38E series (including the ECII), the soundhead detentions once the speed exceeds 30fps. The 16mm soundhead is the same sort of "Davis" tension system as a penthouse reader (just turned 90-degrees). As I said, I would be comfortable with 50fps in either direction but would shy away from going much faster and really watch your ramp speeds.

Take some scrap film, put it in a loop and with weight to give it tension...run it up to whatever speed you want and I'll bet there comes a point (probably before you get to 100fps, that the film starts screaming on the reader sprocket and possibly from the tensioners...it will also likely get scratched on the drum...including when you slow things down and it starts slipping some more.

[Sean McKinnon]

Yes, all of the "e" and "ECII" machines that I have seen with high speed rewinding used the RSSD for Dolby Digital no penthouse reader. However, I have seen ones with DTS readers on them and I would probably reccomend unthreading the DTS reader for high speed reverse. What say you Steve?

[Steve Guttag]

I've never unthreaded a DTS reader for shuttling and have seen film run without issue at 100fps through one. Unlike a Dolby/BACP or SDDS reader, there is no sprocket, no flywheel (nothing of significant mass), no area of the image is in contact with any part so slipping is not an issue and all of the rollers are notably larger than their penthouse counterparts so their angular velocities are slower. I have never looked at one running 300fps though (never put in an ECII).

[Sean McKinnon]

Good to know. I wasn't sure!